The present invention is in the field of biotechnology. More specifically, the invention is in a field often referred to as enzyme engineering, in which through genetic alterations or other means, the amino acid sequences of enzymes of interest are changed in order to alter or improve their catalytic properties. The embodiments of the invention which are described below involve methods in the fields of genetic engineering and enzymology, and more particularly, to the design of protein kinases and other multi-substrate enzymes, including inhibitable such enzymes, and to related materials, techniques and uses.
It is only logical that cell-to-cell communications in a multicellular organism must be fast, and that they must be able to allow cells to respond to one another in diverse and complex ways. Typically, the intracellular signals used are molecules called xe2x80x9cligandsxe2x80x9d and a given ligand can bind to a particular type of receptor on the surface of those cells that are to receive that signal. But this simple ligand binding alone is not enough to provide for the complex responses that the receiving cells may need to make. Cells therefore amplify and add complexity to this signal through complex, often cascading mechanisms leading to the rapid modulation of catalytic activities inside the cell, which in turn can produce complex, and sometimes dramatic, intracellular responses. This process as a whole, from initial ligand binding to completion of the intracellular response, is called xe2x80x9csignal transduction.xe2x80x9d Signal transduction is often accomplished by the activation of intracellular enzymes that can act upon other enzymes and change their catalytic activity. This may lead to increases or decreases in the activity certain metabolic pathways, or may lead to even large intracellular changes, for example, the initiation of specific patterns of gene expression. The ability of one enzyme to alter the activity of other enzymes generally indicates that the enzyme is involved in cellular signal transduction.
The most common covalent modification used in signal transduction process is phosphorylation, which results in the alteration of the activity of those enzymes which become phosphorylated. This phosphorylation is catalyzed by enzymes known as protein kinases, which are often simply referred to as xe2x80x9ckinases.xe2x80x9d
Several key features of the kinases make them ideally suited as signaling proteins. One is that they often have overlapping target substrate specificities, which allows xe2x80x9ccross-talkxe2x80x9d among different signaling pathways, thus allowing for the integration of different signals (1). This is thought to be a result of the need for each kinase to phosphorylate several substrates before a response is elicited, which in turn provides for many types of diverse signaling outcomes. For example, a given kinase may in one instance transmit a growth inhibitory signal and in another instance transmit a growth promoting signal, depending on the structure of the extracellular ligand that has bound to the cell surface (2).
A second key feature is that the kinases are organized into several modular functional regions, or xe2x80x9cdomainsxe2x80x9d (3). One domain known as xe2x80x9cSH3xe2x80x9d is a proline-rich region of 55-70 amino acids in length, and another, known as xe2x80x9cSH2xe2x80x9d is a phosphotyrosine binding region of about 100 amino acids in length. These two domains are believed to be involved in recognizing and binding to the protein substrates. The third domain, xe2x80x9cSH1xe2x80x9d is comprised of about 270 amino acids, and is the domain which is responsible for catalysis. It also contains the binding site for the nucleoside triphosphate which is used as energy source and phosphate donor (3). Other domains, including myristylation and palmitylation sites, along with SH2 and SH3, are responsible for assembling multiprotein complexes which guide the catalytic domain to the correct targets (3,22,23). Molecular recognition by the various domains has been studied using by x-ray diffraction and by using NMR methods (24-28).
These domains appear to have been mixed and matched through evolution to produce the large protein kinase xe2x80x9cfamily.xe2x80x9d As many as 1000 kinases are thought to be encoded in the mammalian genome (4), and over 250 kinases have already been identified. The large number of kinases and the large number of phosphorylation-modulated enzymes that are known to exist inside cells allow for rapid signal amplification and multiple points of regulation.
A third key feature of the kinases is their speed. The kinetics of phosphorylation and dephosphorylation is extremely rapid in many cells (on a millisecond time scale), providing for rapid responses and short recovery times, which in turn makes repeated signal transmission possible (5).
These features of the kinases have apparently led them to be used in a vast array of different intracellular signal transduction mechanisms. For example, growth factors, transcription factors, hormones, cell cycle regulatory proteins, and many other classes of cellular regulators utilize tyrosine kinases in their signaling cascades (12,13). Tyrosine kinases catalytically attach a phosphate to one or more tyrosine residues on their protein substrates. The tyrosine kinases include proteins with many diverse functions including the cell cycle control element c-abl (14-16), epidermal growth factor receptor which contains a cytoplasmic tyrosine kinase domain (12), c-src, a nonreceptor tyrosine kinase involved in many immune cell functions (13), and Tyk2, a cytoplasmic tyrosine kinase which is involved in phosphorylation of the p91 protein which is translocated to the nucleus upon receptor stimulation and functions as a transcription factor (17). The serine/threonine kinases make up much if not all of the remainder of the kinase family; these catalytically phosphorylate serine and threonine residues in their protein substrates, and they have similarly diverse roles. They share homology in the 270 amino acid catalytic domain with tyrosine kinases. As such, although the discussion which follows focuses more particularly on the tyrosine kinases, that discussion is generally applicable to the serine/threonine kinases as well.
Unfortunately, the very features which make kinases so useful in signal transduction, and which has made them evolve to become central to almost every cellular function, also makes them extremely difficult, if not impossible, to study and understand. Their overlapping protein specificities, their structural and catalytic similarities, their large number, and their great speed make the specific identification of their in vivo protein substrates extremely difficult, if not impossible, using current genetic and biochemical techniques. This is today the main obstacle to deciphering the signaling cascades involved in tyrosine kinase-mediated signal transduction (4,6-8).
Efforts to dissect the involvement of specific tyrosine kinases in signal transduction cascades have been frustrated by their apparent lack of protein substrate specificity in vitro and in vivo (4,8). The catalytic domains of tyrosine kinases possess little or no inherent protein substrate specificity, as demonstrated by domain swapping experiments (18-23). The catalytic domain from one tyrosine kinase can be substituted into a different tyrosine kinase with little change in the protein substrate specificity of the latter (22).
The poor in vitro specificity of kinases also makes it difficult, if not impossible, to extrapolate what the in vivo function of given kinases might be. An isolated tyrosine kinase of interest will often phosphorylate many test protein substrates with equal efficiency (29). This apparently poor substrate specificity is also found in vivo; for example, many genetic approaches, such as gene knock out experiments, give no interpretable phenotype due to compensation by other cellular tyrosine kinases (30,31).
Another complication is that many tyrosine kinases have been proposed to phosphorylate downstream and upstream proteins which are themselves tyrosine kinases; although this appears to make complex positive feedback loops possible, it also makes dissecting the cascade even more difficult (1).
One important avenue for deciphering the role and understanding the function of enzymes, both in vitro and in vivo, is the use of specific enzyme inhibitors. If one or more compound can be found that will inhibit the enzyme, the inhibitor can be used to modulate the enzyme""s activity, and the effects of that decrease can be observed. Such approaches have been instrumental in deciphering many of the pathways of intermediary metabolism, and have also been important in learning about enzyme kinetics and determining catalytic mechanisms.
In addition, such inhibitors are among the most important pharmaceutical compounds known. For example, aspirin (acetylsalicylic acid) is such an inhibitor. It inhibits an enzyme that catalyzes the first step in prostaglandin synthesis, thus inhibiting the formation of prostaglandins, which are involved in producing pain (72). Traditional drug discovery can be characterized as the design and modification of compounds designed specifically to bind to and inactivate a disease-causing protein; the relative success of such an effort depends upon the selectivity of the drug for the target protein and its lack of inhibition of non-disease associated enzymes with similar enzyme activities.
Such approaches would appear to be promising ways to develop treatments for cancer, since many human cancers are caused by disregulation of a normal protein (e.g., when a proto-oncogene is converted to an oncogene through a gene translocation). And since kinases are key regulators, they have turned out to be very common proto-oncogenes, and thus ideal drug design targets.
The process of designing selective inhibitors is relatively simple in cases where few similar enzymes are present in the target organism, for example in cases where inhibitors of a protein unique to bacteria can be targeted. But unfortunately, the similarities between the kinases and their large number has almost completely frustrated the discovery and design of specific inhibitors, and has blocked most hopes of developing specific pharmaceutical treatments aimed at the proto-oncogene level. It is expected that the vast majority of candidate inhibitors will inhibit multiple kinases, even though they may have initially been identified as inhibiting a particular, purified kinase.
This is not to say, however, that inhibitors with at least some degree of kinase specificity cannot be found. Several natural products have been identified which are relatively specific for particular kinase families, but attempts to derive general rules about kinase inhibition based on these has failed. Furthermore, as the following examples show, specificity in most cases is quite limited. For example, the compound Damnacanthal was reported to be a xe2x80x9chighly potent, selective inhibitorxe2x80x9d of the kinase p561ck (73); as shown in FIG. 10A, this compound has an inhibition constant (IC50) for that kinase which is almost seven times lower than for the kinase src (the IC50 is the concentration of inhibitor which must be added to reduce catalytic activity by 50%). The compound PPI (FIG. 10B) has a binding affinity for the kinase Ick which is very strong (IC50=0.005 xcexcM); but unfortunately, the inhibition of other kinases of the src family is very similar. It inhibits the kinase fyn with an almost identical IC50, 0.006 xcexcM, and has only about a 4-fold higher IC50 for the kinase hck (IC50=0.020 xcexcM). The compound CGP 57148 (FIG. 10C) has been reported to be xe2x80x9csemi-selectivexe2x80x9d for the kinases abl (IC50=0.025 xcexcM) and PDGFR (IC50=0.030 xcexcM) (74). Nevertheless, considering the vast number of kinases and their relative cellular importance, and also considering that the above-described inhibitors have only been reported in the last two years, it appears that success in discovering or designing selective kinase inhibitors has been remarkably limited.
These difficulties described above have implications well beyond the mere frustration of scientists; they have frustrated efforts to decipher the kinase cascades and the function of individual kinases in those cascades and other cellular mechanisms. Such an understanding of kinase activity and function may be essential before certain human diseases can be effectively treated, prevented or cured. For example, it has been known for over thirty years that the oncogene bcr-abl is a protein kinase that is responsible for chronic myelogenous leukemia; but the physiological substrates that it acts upon to cause oncogenesis, which may be important drug design targets, have yet to be definitively identified (11). On the bright side, despite this shortcoming, the above-described inhibitor CGP 57148 is reportedly now undergoing clinical trials for use in treating myelogenous leukemia, even though the substrates it may block phosphorylation of in vivo are not known.
The medical significance of these difficulties is further illustrated by the Rous sarcoma virus (RSV), which has become an important model system for studying the role of kinases in oncogenesis. RSV transformation of fibroblasts is controlled by a single viral gene product, the protein tyrosine kinase v-src (32). It is the rapid time course and the dramatic morphological changes during RSV fibroblast transformation that have made RSV a paradigm for studies of oncogene activity in all cells. The origin (33), regulation (3,8,34,35), and structure (25,27,36) of v-Src have been extensively studied and are well understood (8,37,38). But central questions about this intensely studied kinase remains unanswered: what are its direct cellular substrates? Does inhibition of its catalytic activity effectively inhibit, or even reverse, transformation? Would such inhibition be an effective therapy for or prophylactic against RSV transformation? Unfortunately, as discussed above, the answers to these questions are not forthcoming, largely because the number of cellular kinases is enormous (it is estimated that 2% of the mammalian genome encodes protein kinases (4)) and because tyrosine kinases display overlapping substrate specificities (8,39) and share catalytic domains, making the design of specific inhibitors enormously difficult.
The expression of v-Src in fibroblasts results in the tyrosine phosphorylation of over fifty cellular proteins (37). These same substrates are also phosphorylated by other kinases in untransformed fibroblasts (40). Even the most sophisticated biochemical and genetic techniques, including anti-phosphotyrosine protein blots of transformed fibroblasts, transfection of fibroblasts with transformation-defective v-Src mutants, temperature-sensitive v-Src mutants, gene knock-out studies of cellular Src host-range dependent Src mutants, anti-v-Src immunoprecipitation, and use of kinase specific inhibitors, have not led to the unambiguous identification of direct substrates for v-Src (see reference (38) for a comprehensive review). But this situation is not unique; in fact, the direct substrates for the majority of cellular kinases remain unidentified (8). Furthermore, as discussed above, there also are remarkably few compounds known to selectively inhibit individual kinases, or even groups of related kinases.
Although the forgoing difficulties are daunting, new methods of rational drug design and combinatorial organic synthesis make the design or discovery of kinase-specific inhibitors feasible given sufficient resources. However, because the kinase networks are highly degenerate and interconnected in unknown ways, there is considerable uncertainty with regard to many diseases which kinases should be targeted for inhibition. Moreover, it is by no means clear that a specific inhibitor of a given kinase will have any effect on the disease, either in vitro or in vivo. Because kinases can be highly promiscuous, there is a significant chance that inhibiting one kinase will simply force another kinase to xe2x80x9ctake its place.xe2x80x9d Therefore, there is a need for a simple and direct way to determine the biochemical and cellular effects of inhibiting a given kinase, before herculean efforts are undertaken to design or discover specific inhibitors.
From the forgoing, it is clear that there has been a long felt but unsatisfied need for ways to identify which cellular proteins are acted upon by individual protein kinases. Such a method would ideally also allow for the quantitative measurement of relative activity of a given kinase on its protein substrates, which could be used, for example, to detect how or whether actual or potential drug compounds might modulate kinase activity. In addition, there has also been a need for specific inhibitors of individual kinases or kinase families, which could be used to identify protein substrates (by looking for which proteins are not phosphorylated or are more weakly phosphorylated in the presence of the inhibitor), to study the biochemical and phenotypic effects of rapidly down-regulating a given kinase""s activity, for use as drugs to treat kinase-mediated diseases, and to confirm that tedious efforts to design or develop more traditional inhibitor drugs would be worthwhile. As is described in considerable detail below, the present invention for the first time provides a method for the highly specific inhibition of individual kinases, which have been engineered to bind the inhibitor more readily than the wild-type form of that kinase or other, non-engineered kinases. The invention also provides for the engineered kinases and the inhibitors to which they are adapted.
Moreover, as will become apparent, this method is even more broadly applicable, as it would provide similar advantages for the study of other enzymes which, like the kinases, covalently attach part of at least one substrate to at least one other substrate.
The present invention involves the engineering of kinases and other multi-substrate enzymes such that they can become bound by inhibitors which are not as readily bound by their wild-type forms. Modified substrates and mutant enzymes that can bind them have been used to study an elongation factor (41) and a receptor for cyclophilin A (42). However, prior to the present invention, it was not known how, or even if, multi-substrates enzymes which covalently attach part or all of a donor substrate onto a recipient substrate could be engineered to bind to an inhibitor, yet still retain at least some catalytic activity and at least some specificity for the recipient substrate in the absence of the inhibitor. The present invention is that this can be done, as explained below; and this invention for the first time opens the door to the selective inhibition of individual kinases, which are not only important tools for understanding of the kinase cascades and other complex catalytic cellular mechanisms, but also may provide avenues for therapeutic intervention in diseases where those mechanisms come into play.
The present invention provides a solution to the above-described problems by providing materials and methods by which a single protein kinase can be specifically inhibited, without the simultaneous inhibition of other protein kinases.
In a first aspect, the present invention involves the engineering of kinases and other multi-substrate enzymes such that they can utilize modified substrates which are not as readily used by their wild-type forms. The invention further provides such chemically modified nucleotide triphosphate substrates, methods of making them, and methods of using them. The methods of the present invention include methods for using the modified substrates along with the engineered kinases to identify which protein substrates the kinases act upon, to measure the extent of such action, and to determine if test compounds can modulate such action.
In a further aspect, the invention provides engineered protein kinases which can bind inhibitors that are not as readily bound by the wild-type forms of those enzymes. Methods of making and using all such engineered kinases are also provided. The invention further provides such inhibitors, methods of making them, and methods of using them. The methods of the present invention include methods for using the inhibitors along with the engineered kinases to identify which protein substrates the kinases act upon, to measure the kinetics of such action, and to determine the biochemical and cellular effects of such inhibition. They also relate to the use of such inhibitors and engineered kinases to elucidate which kinases may be involved in disease; these kinases can then become the subject of efforts to design or discover more traditional specific inhibitors of their wild-type forms, which may prove to be valuable in treating the kinase-related disease or disorder.
Furthermore, methods are provided for inserting the engineered kinase into cells or whole animals, preferably in place of the corresponding wild-type kinase, and then using the inhibitor to which it has been adapted as a tool for study of the disease kinase relationship, and ultimately, as a drug for the treatment of the disease.
The present invention also more generally relates to engineered forms of multi-substrate enzymes which covalently attach part or all of at least one (donor) substrate to at least one other (recipient) substrate. These engineered forms will accept modified substrates and inhibitors that are not as readily bound by the wild-type forms of those enzymes.
The invention also relates to methods for making and using such engineered enzymes, as well as the modified donor substrates. The methods of the present invention include methods for using the modified substrates and inhibitors along with the engineered enzymes to identify which substrates the enzymes act upon, to measure the kinetics of such action, and in the instance of the modified substrates, to determine the recipient substrates to which part or all of the donor substrate becomes attached, to measure the extent of such action, and to identify and measure the extent of modulation thereof by test compounds.
In the instance of inhibitors, the methods seek to determine the biochemical and cellular effects of such inhibition. The methods also extend to the use of such inhibitors and engineered enzymes to elucidate which enzymes may be involved in disease; these enzymes can then become the subject of efforts to design or discover specific inhibitors of their wild-type forms, which may prove to be valuable in treating the enzyme-related disease or disorder. Furthermore, methods are provided for inserting the engineered enzyme into cells or whole animals, preferably in place of the corresponding wild-type enzyme, and then using the inhibitor to which it has been adapted as a tool for study of the disease-enzyme relationship, and ultimately, as a drug for the treatment of the disease.
According to the present invention, through enzyme engineering a structural distinction can be made between the nucleotide binding site of a protein kinase of interest, and the nucleotide binding sites of other kinases. This distinction allows the engineered kinase to use a nucleotide triphosphate or an inhibitor that is not as readily bound by the wild-type form of that kinase, or by other kinases. In a preferred embodiment with respect to the inhibitor, the inhibitor used is one that is xe2x80x9corthogonalxe2x80x9d to the xe2x80x9cnaturalxe2x80x9d nucleotide triphosphate substrate for that kinase, or is orthogonal to a less specific inhibitor (e.g., one which is readily bound by the wild-type form of that kinase). The term xe2x80x9corthogonalxe2x80x9d as further discussed below, means that the substrate or inhibitor is similar in structure (including those that are geometrically similar but not chemically similar, as described below), but differs in a way that limits its ability to bind to the wild-type form.
An engineered kinase made according to the present invention will be able to use an orthogonal nucleotide triphosphate substrate that is not as readily used by other, nonengineered kinases present in cells. Preferably, it will be able to use an orthogonal nucleotide triphosphate that is not substantially used by other kinases; and most preferably, it will be able to use an orthogonal nucleotide triphosphate substrate that can not be used at all by other kinases. By labeling the phosphate on the orthogonal substrate, e.g., by using radioactive phosphorous (32P), and then adding that labeled substrate to permiabilized cells or cell extracts, the protein substrates of the engineered kinase will become labeled, whereas the protein substrates of other kinases will be at least labeled to a lesser degree; preferably, the protein substrates of the other kinases will not be substantially labeled, and most preferably, they will not be labeled at all.
The detailed description and examples provided below describe the use of this strategy to uniquely tag the direct substrates of the prototypical tyrosine kinase v-Src. Through protein engineering a chemical difference has been made in the amino acid sequence which imparts a new structural distinction between the nucleotide binding site of the modified v-Src and that of all other kinases. The v-Src kinase Applicant has engineered recognizes an ATP analog (A*TP), N6(cyclopentyl)ATP, which is orthogonal to the nucleotide substrate of wild-type kinases. The generation of a v-Src mutant with specificity for an orthogonal A*TP substrate allows for the direct substrates of v-Src to be uniquely radiolabeled using [xcex3-32P] N6(cyclopentyl)ATP, because it is able to serve as substrate to the engineered v-Src kinase, but is not substantially able to serve as substrate for other cellular kinases.
The detailed description and examples provided below describe the use of this strategy to uniquely identify the direct substrates of the prototypical tyrosine kinase, v-Src. Through protein engineering a chemical difference has been made in the amino acid sequence which imparts a new structural distinction between the nucleotide binding site of the modified v-Src and that of all other kinases. The engineered v-Src kinases that have been made and presented herein bind to an orthogonal analog of the more general kinase inhibitor PP3: the compound N4 cyclopentoyl PP3. The generation of a v-Src mutant with specificity for such an inhibitor allows for the mutant to be inhibited, whereas other kinases in the same test system are not substantially inhibited, not even the wild-type form of that same kinase.
As is apparent from the forgoing, it is one object of the present invention to provide a mutant protein kinase which accepts an orthogonal nucleotide triphosphate analog as a phosphate donor substrate.
Another object of the present invention to provide a nucleotide sequence which encodes such a mutant protein kinase; and it is a further object to provide a method for producing such a nucleic acid sequence.
It is also an object of the invention to provide methods for producing such a mutant protein kinase, for example, by expressing such a nucleic acid sequence.
It is also an object of the present invention to provide such orthogonal nucleotide triphosphates and methods for their synthesis, including N6(cyclopentyl)ATP, N6(cyclopentyloxy)ATP, N6(cyclohexyl)ATP, N6(cyclohexyloxy)ATP, N6(benzyl)ATP, N6(benzyloxy)ATP, N6(pyrolidino)ATP and N6(piperidino)ATP (27).
It is yet another object of the invention to provide a method for determining whether a test compound positively or negatively modulates the activity of a protein kinase with respect to one or more protein substrates.
More particularly, and in accordance with the further aspect of the invention, it is a primary object provide a mutant protein kinase which binds to and is inhibited by an inhibitor, which inhibitor less readily binds to or inhibits the corresponding wild-type kinase.
A further object of the present invention is to provide a nucleotide sequence which encodes such a mutant protein kinase; and it is a further object to provide a method for producing such a nucleic acid sequence.
It is also an object of the invention to provide methods for producing such a mutant protein kinase, for example, by expressing such a nucleic acid sequence.
It is another object of the present invention to provide such inhibitors, such as the compound N4(cyclopentoyl)PP3, and methods for their synthesis.
Another object is to provide a method for determining what are the substrates for a given protein kinase.
It is yet another object of the invention to provide a method for determining whether specific inhibition of a particular kinase produces a biochemical or phenotypic effect in a test system such as a cell-free extracts, cell cultures, or living multicellular organisms.
It is a further object of the invention to provide a method to determine whether inhibition of a particular kinase might have therapeutic value in treating disease.
It is yet another object to provide methods for the study of the activity, kinetics, and catalytic mechanisms of a kinase by studying the inhibition of the corresponding mutant of the present invention.
A further object is to provide a methods of preventing and treating kinase-mediated diseases by introducing an inhibitor-adapted mutant kinase of the present invention into a diseased organism, and preferably diminishing or, most preferably, depleting the organism of the wild-type enzyme; and then administering the inhibitor to regulate the activity of the now disease-mediating mutant kinase so as to diminish or eliminate the cause or symptoms of the disease.
Based upon the forgoing and the detailed description of the present invention provided below, one of ordinary skill in the art will readily recognize that the present invention can be used more generally to study multi-substrate enzymes which covalently transfer a donor substrate or portion thereof to a recipient substrate, as do the kinases. Such applications of the present invention are also further described in the detailed description which follows.
Accordingly, it is yet a further object of the present invention to provide a mutant multi-substrate enzyme which binds to an inhibitor, which inhibitor is less readily bound to the wild-type enzyme or to other enzymes with similar activity.
It is another object of the invention to provide a nucleotide sequence which encodes such a mutant multi-substrate enzyme; and it is a further object to provide a method for producing such a nucleic acid sequence.
It is also an object of the invention to provide methods for producing such a mutant multi-substrate enzyme, for example, by expressing such a nucleic acid sequence.
It is also an object of the present invention to provide such inhibitors and methods for their synthesis.
Another object is to provide a method for determining what are the substrates for a given multi-substrate enzyme.
It is yet another object of the invention to provide a method for determining whether specific inhibition of a particular multi-substrate enzyme produces a biochemical or phenotypic effect in a test system such as a cell-free extract, cell culture, or living multicellular organism.
It is a further object of the invention to provide a method to determine whether inhibition of a particular multi-substrate enzyme might have therapeutic value in treating disease.
It is yet another object to provide methods for the study of the activity, kinetics, and catalytic mechanisms of a multi-substrate enzyme by studying the inhibition of the corresponding mutant of the present invention.
A further object is to provide a methods of preventing and treating multi-substrate enzyme-mediated diseases by introducing an inhibitor-adapted multi-substrate enzyme of the present invention into a diseased organism, and preferably diminishing or, most preferably, depleting the organism of the wild-type enzyme; and then administering the inhibitor to regulate the now disease-mediating mutant enzyme so as to diminish or eliminate the cause or symptoms of the disease.
These and other objects of the present invention will, from the detailed description, examples and claims set forth below, become apparent to those of ordinary skill in the art.